Common mode rejection (CMR) is the ability of a differential amplifier or balanced audio circuit to reject signals that are common to both input lines, such as interference or noise induced by external electrical sources.
In balanced audio connections, two signal conductors carry identical signals with opposite polarity. Any external noise picked up along the cable appears equally on both lines. When the differential amplifier at the input stage processes the signal, it amplifies the difference between the two while cancelling any common noise component. The effectiveness of this cancellation is measured as the common mode rejection ratio (CMRR), typically expressed in decibels (dB).
High CMRR values indicate better noise rejection and cleaner signal transmission, especially over long cable runs or in electrically noisy environments such as recording studios or live sound setups. Maintaining proper cable balance, impedance matching, and high-quality input circuitry is essential to achieving strong common mode rejection and ensuring professional-grade audio performance.
